MIC2776 Micrel Semiconductor, MIC2776 Datasheet - Page 7

• Manufacturer Part Number: MIC2776
• Description: Micro-Power Low Voltage Supervisor
• Manufacturer: Micrel Semiconductor

Interfacing to Processors With Bidirectional Reset Pins
Some microprocessors have reset signal pins that are bidi-
rectional, rather than input only. The Motorola 68HC11 family
is one example. Because the MIC2776N’s output is open-
drain, it can be connected directly to the processor’s reset pin
using only the pull-up resistor normally required. See Figure 2.
Transient Response
The MIC2776 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the voltage
threshold without tripping the output.
As shown in Figure 3, the narrower the transient, the deeper
the threshold overdrive that will be ignored by the MIC2776.
The graph represents the typical allowable transient duration
for a given amount of threshold overdrive that will not gener-
ate a reset.
September 29, 2000
MIC2776
Figure 2. Interfacing to Bidirectional Reset Pin
1.0V ±5%
2.5V ±5%
Figure 3. Typical INPUT Transient Response
V
V
V
CORE
I/O
CC
Figure 1. MIC2776 Example Design
Manual
Reset
R1
R2
RESET COMP. OVERDRIVE, V
R1
676k
1%
40
35
30
25
20
15
10
R2
324k
1%
5
0
0
VDD
IN
/MR
MIC2776N
Transient Response
VDD
IN
/MR
MIC2776
Typical INPUT
/RST
GND
100
/RST
GND
100k
200
MICROPROCESSOR
MICROPROCESSOR
REF
VCC
/RESET
–V
VCORE
/RESET
VI/O
300
IN
(mV)
GND
GND
7
Ensuring Proper Operation at Low Supply
At levels of V
driver cannot turn on sufficiently to produce a valid logic-low
on the /RST output. In this situation, other circuits driven by
/RST could be allowed to float, causing undesired operation.
(In most cases, however, it is expected that the circuits driven
by the MIC2776L will be similarly inoperative at V
If a given application requires that /RST be valid below V
= 1.2V, this can be accomplished by adding a pull-down
resistor to the /RST output. A value of 100kΩ is recom-
mended as this is usually an acceptable compromise of
leakage current and pull-down current. The resistor’s value is
not critical, however. See Figure 4.
The statements above also apply to the MIC2776H’s RST
output. That is, to ensure valid RST signal levels at V
1.2V, a pull-up resistor (as opposed to a pull-down) should be
added to the RST output. A value of 100kΩ is typical for this
application as well. See Figure 5.
V
Figure 4. MIC2776L Valid /Reset Below 1.2V
CC
Figure 5. MIC2776H Valid Reset Below 1.2V
V
CC
Manual
Reset
Manual
Reset
DD
R1
R2
R1
R2
below 1.2V, the MIC2776L’s /RST output
VDD
IN
/MR
MIC2776H
VDD
IN
/MR
MIC2776L
GND
RST
/RST
GND
100k
Rpull-up
100k
Rpull-down
MICROPROCESSOR
MICROPROCESSOR
VCC
/RESET
VCC
RESET
GND
GND
DD
MIC2776
≤ 1.2V.)
Micrel
DD
DD
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